1. Field of the Invention
The present invention relates to a method for producing a silicon single crystal wafer in high productivity wherein a size of crystal defect inside the crystal, called grown-in defect, is decreased by doping nitrogen when pulling a silicon single crystal by a Czochralski method (hereinafter referred to as "CZ method"), and gettering ability is improved by subjecting the wafer to heat treatment, and the silicon single crystal wafer produced by the method.
2. Description of the Related Art
As a wafer for fabrication of a device such as a semiconductor integrated circuit, a silicon single crystal wafer grown by a Czochralski method (CZ method) is mainly used. If crystal defects are present in such a silicon single crystal wafer, pattern failure is caused when a semiconductor device is fabricated. Particularly, the pattern width of devices which is highly integrated in recent years is very fine as 0.35 .mu.m or less. Accordingly, even small crystal defects as 0.1 .mu.m may cause defects such as pattern failures in the device, and may remarkably lower a yield and characteristics of the device. Accordingly, the crystal defects in the silicon single crystal wafer have to be decreased as thoroughly as possible.
Recently, it has been reported that the above-mentioned crystal defects called grown-in defect incorporated during growth of the crystal are found in the silicon single crystal grown by CZ method by various measurement methods. For example, these crystal defects in a single crystal grown at a general growth rate in commercial production (for example, about 1 mm/min or more) can be detected as a pit by subjecting the surface of the crystal to preferential etching (Secco etching) with Secco solution (a mixture of K.sub.2 Cr.sub.2 O.sub.7, hydrofluoric acid and water) (See Japanese Patent Application Laid-open (kokai) No. 4-192345).
The main cause of generation of such a crystal defect is considered to be a cluster of vacancies which are aggregated during manufacture of single crystal or an oxide precipitate which is an agglomerate of oxygen atoms getting in from a quartz crucible. When these crystal defects are present in the surface portion in which a device is fabricated, they come to harmful defect to degrade characteristics of the device. Accordingly, it is desirable to reduce these crystal defects to produce the wafer having a sufficiently deep denuded zone (DZ) in the surface layer.
When heavy metal impurity such as Fe, Cu or the like is present in the surface portion of the silicon single crystal wafer, the characteristics of the device may be degraded during fabrication thereof. Accordingly, it is important to remove heavy metal impurity by intrinsic gettering (IG) wherein the bulk micro-defects are precipitated as a gettering site in a bulk portion of the silicon wafer. It is necessary to generate the bulk micro-defects (BMD) in sufficient density in the bulk portion of the wafer in order to make the intrinsic gettering effective.
The bulk micro-defect used herein means micro-defects such as oxide precipitates, dislocation, stacking fault or the like induced by oxide precipitates, which are present in the bulk portion.
From the above-mentioned points of view, for manufacture of silicon semiconductor wafer, the depth of denuded zone in the surface part of the wafer in which a device is fabricated and the density of bulk micro-defects in the bulk portion of the wafer which is to be a gettering site after gettering heat treatment or device process heat treatments are important.
It has been known that the depth of denuded zone and the density of bulk micro-defects depend on interstitial oxygen concentration in the silicon single crystal grown by CZ method or cooling rate (growth rate) during growth of silicon single crystal. Accordingly, the depth of denuded zone and the bulk micro-defect density of the silicon wafer have been controlled mainly by controlling oxygen concentration and cooling rate.
For example, it is known that a density of the above-mentioned cluster of vacancies can be lowered by decreasing a growth rate of the crystal extremely (for example, to 0.4 mm/min or less) (See Japanese Patent Application Laid-open (kokai) No.2-267195). However, adopting this method there is generated a crystal defect which is considered to be a dislocation loop formed as a result of new aggregation of excess interstitial silicon atoms, which may degrade characteristics of a device significantly. Accordingly, the problem cannot be solved by the method. Furthermore, productivity of the single crystal and cost performance are extremely decreased in the method, since the growth rate of the crystal is decreased from about 1.0 mm/min as usual or more to 0.4 mm/min or less.
In order to reduce crystal defects due to oxide precipitate in the surface portion of the wafer, there is a method of growing crystal with lowering initial oxygen concentration in the crystal. However, in the method, oxygen concentration, and amount of precipitated oxygen are lowered not only in the surface portion in which the device is fabricated, but also in the bulk of the wafer. When amount of precipitated oxygen is lowered in the bulk portion, intrinsic gettering (IG) effect wherein harmful heavy metal or the like is captured in device process cannot be achieved, resulting in lowering a yield in device fabrication.
Furthermore, in the silicon single crystal wafer obtained from the silicon single crystal ingot grown by such a method wherein oxygen concentration and cooling rate are controlled, the size of crystal defect such as grown-in defect or the like is large, and thus it cannot be eliminated sufficiently by a gettering heat treatment conducted thereafter. As a result, the depth of denuded zone of conventional silicon single crystal wafer gets shallow as 0.5 .mu.m at most.
In the conventional method, although the density of bulk micro-defect after heat treatment of the wafer having high oxygen concentration as 20 ppma (JEIDA) is about 1.times.10.sup.10 number/cm.sup.3 (defect/cm.sup.3), which results crystal defects due to oxygen remain near the surface, resulting in lowering of yield of the device. Furthermore, the density of bulk micro-defect after heat treatment of the wafer having oxygen concentration of 9 to 17 ppma (JEIDA) which is generally used for the device is about 1.times.10.sup.9 number/cm.sup.3 at most, which is insufficient to make intrinsic gettering effective. Accordingly, there is a problem of lowering of device yield due to heavy metal impurity on the surface of the wafer.